1. Field of the Invention
The present invention relates to a photomask, and more particularly to a photomask specifically suitable for use in a reduction projection exposing method for selectively exposing a photoresist film with a light beam with partially different intensity.
2. Description of the Prior Art
As is well known, various fine patterns in a magnetic bubble memory element and in semiconductor devices of various kinds are formed through the so-called photolithography.
The photolithography is useful for the formation of various fine patterns, but involves the following problems. For example, in the case where different patterns such as conductor and permalloy patterns in a magnetic bubble memory element are to be formed, it is required to use a plurality of photomasks, the number of which is equal to the number of kinds of patterns, and to carry out exposure and development for the same number of times. Accordingly, the pattern forming process is very cumbersome, and moreover it is very difficult to form each of fine patterns at its predetermined accurate position because a mask alignment error is inevitably accompanied at each mask alignment.
In order to solve such problems, a method has been proposed in which a photoresist film is irradiated with light in such a manner that the intensity of light incident upon a portion of the photoresist film is made different from the intensity of light incident upon another portion by using a mask having portions different in light transmissivity, in order to form a plurality of different patterns with a single photomask and one exposure (the above method is herein referred to as one-mask method).
In more detail, the photoresist film is exposed to light through a photomask such as shown in FIG. 1, which is provided with a mask pattern formed on a transparent quartz plate 1 and having a semi-transparent portion 2 and an opaque portion 3, and is then developed.
In the case where the photoresist film having been exposed to light is of positive type, the solubility of the exposed photoresist film is increased in accordance with the quantity of light having illuminated the photo-resist film. Accordingly, as shown in FIG. 2, a resist pattern 4 which has a cross section corresponding to that of the mask pattern shown in FIG. 1, is formed on a substrate 5 which is to be processed. When the photo-resist pattern formed in the above-mentioned manner and having portions different in thickness is used to form, for example, a magnetic bubble memory element, the following features are obtained which are not found in conventional methods.
That is, as shown in FIG. 3a, the resist pattern 4 having portions of different thickness is formed on a laminated structure. In the laminate, a monocrystalline magnetic garnet film 7 capable of holding magnetic bubbles, a first insulating film 8, a conductive film 9, a second insulating film 10 and a permalloy film 11 (namely, an alloy of iron and nickel) have been successively piled on a monocrystalline nonmagnetic garnet substrate 6.
Thereafter, dry etching is carried out for the above-mentioned laminated structure provided with the resist pattern 4. Then, as shown in FIG. 3b, the laminated structure has substantially the similar form as the resist pattern 4 in cross section, since the cross sectional form of the laminated structure obtained after the dry etching corresponds to that of the resist pattern 4. Thus, according to the one-mask method, a permalloy pattern 11 and a conductor pattern 9 are formed by exposing the photoresist film to light only once while using only one mask. Accordingly, there is no fear of an error arising due to mask alignment, and therefore a very accurate positional relation is obtained between the permalloy pattern 11 and the conductor pattern 9.
As mentioned above, the one-mask method has the advantages that the fabricating process is simple and that two or more kinds of patterns can be formed with a very accurate positional relation, but has a drawback that the fabrication of the photomask is cumbersome.
In order to form a mask pattern having portions of different light transmissivity, it has been required to form a plurality of patterns successively on a substrate through evaporation and photoetching techniques. Since the masking layer is formed through evaporation techniques, it is difficult to partially vary the thickness of a semi-transparent film, and further it is difficult to finely adjust the quantity of light passing through the semi-transparent film.